STUDIES OF MONOAMINE OXIDASE: (1) Brain mitochondria (ESR Studies); We have prepared two spin-labeled probes of the enzyme; spin-labeled hydroxyamphetamine (SHA) and spin-labeled hydroxypargyline (SHP). Our preliminary evidence with the SHP indicated that it, like pargyline itself, bound covalently to the flavin moiety of the enzyme. More importantly however, the spin resonance data indicated that the SHP competed with substrates for the active site of the enzyme before covalently binding to the flavin. (Developmental studies of MAO in rat brain): We have determined the developmental characteristics of the enzyme at birth through 40 days of age. Because of the differential effect of oxygen saturation on various substrate affinities of the enzyme, we have measured the effect of oxygen during the developmental periods using different substrates. It seems clear, at the present time, that the effect of oxygen increases with developmental time as well as varying with substrate. Further, we have found that the A-form of the enzyme appeared before that of the B-form. PLATELET STUDIES: (2) We have investigated the nature of MAO in different human blood platelet fractions derived from different density gradients. The forms of MAO (as measured with different substrates and inhibitors appeared to be different in the different platelet bands.